A cationic type photopolymerization initiator is widely known as a catalyst for polymerization of, for example, an epoxy compound and a vinyl ether compound, by irradiation with UV, deep UV, electron beams, X-rays, radioactive rays and the like.
As a cationic type photopolymerization initiator, for example, a sulfonium salt such as triallylsulfonium hexafluoroantimonate (see U.S. Pat. No. 4,058,401) or a 4-(phenylthio)phenyldiphenylsulfonium salt compound (see U.S. Pat. No. 4,173,476), an iodonium salt such as diphenyliodonium hexafluorophosphate or diphenyliodonium hexafluoroantimonate (see e.g. JP-A-50-151996, JP-A-60-47029, etc.) and the like are known.
However, these compounds have low acid generation efficiency, and therefore use of them as a cationic photopolymerization initiator makes it difficult to form a polymer with high hardness.
It is also known that a sulfonium salt and an iodonium salt, having, for example, hexafluorophosphate (PF6−) or tetrafluoroborate (BF4−) as a counter anion show more remarkably reduced photocuring than those having hexafluoroantimonate (SbF6−). However, due to strong toxicity of SbF6− and probable banning of its use in the future, it is required to study a cation part with a new structure and enhanced photo degradability to develop an onium salt with sufficient curing function even with PF6−, BF4− and the like as a counter anion.
Furthermore, in accordance with the recent trend of higher density integration in semiconductor elements, wavelengths of light sources for irradiation instruments used in fine processing, particularly those used in lithography, become shorter and shorter, and in compliance with this trend, chemically amplified resist compositions, wherein an action of an acid generated from an acid generator as a photo sensitive compound is utilized, have generally been used. Further, a sulfonium salt and an iodonium salt have been used also as an acid generator for these chemically amplified resist compositions.
However, a sulfonium salt and an iodonium salt have low solubility in a solvent for resist such as propylene glycol monomethyl ether acetate (PGMEA) or ethyl lactate, and are hardly dispersed uniformly in a resist. These cause a problem of poor semiconductor device, because the poor solubility not only makes difficult to develop sufficiently high resist sensitivity, but also gives rough profiles or side walls of patterns, resulting in a fall of a resist ingredient forming a pattern on a substrate on etching, or collapsing of a pattern itself, to fail to apply specified etching.
The present invention has been completed under such circumstances as mentioned above, and the theme of the invention is to provide a new hybrid type onium salt having an iodonium salt and a sulfonium salt in the molecule, which can be used more practically as a cationic type photopolymerization initiator and an acid generator for a resist and the like.